the manual say nothing about the blue light. The only interesting item is, there was a difference in the circuit between military and civilian vehicles. As far as I could trace, the blue lamp was always standard.
The purpose of the blue light was to reduce the lighting of the number plate. Blue lights were also used e.g. in air raid shelters and U-Boots because it was very suitable for the eyes.
Maybe sometimes clear lamps were used when blue lamps were short on supply.